Spectral analysis using Cisco Clean Air

By | June 4, 2016


Spectrum analysis is a must have tool for a Wi-Fi engineer, it can be used for troubleshooting, look for interference caused by other technologies that use the same frequencies and also to measure the noise floor at a location before deploying Wi-Fi.  Also its very important to know that a Wireless NIC cannot be used to find out the noise floor or other interference. NIC has an Antenna that blocks RF except 2.4 and 5 GHz which are our Wi-Fi frequencies, but there are lots of technologies that use same frequency band so we have a modulation filter that blocks all the RF that is not modulated for 802.11. To do spectrum analysis you need a special card which is able to see all of RF on 2.4 and 5 GHz.

A Cisco clean air AP can be used to do spectrum analysis. For this set up we will need to have the AP with an autonomous image on it and then set it up in a spectrum mode. All the Cisco APs after 3500 series come with clean air and has dual radio (2.4 and 5 GHz). At same time we can run two instances of Cisco spectrum expert and can capture spectrum for both the frequencies.

Hardware and Software Requirement

  1. Cisco AP (with Clean Air)
  2. Laptop
  3. Battery
  4. Power Injector
  5. Cisco Autonomous Image
  6. Cisco Spectrum Expert Version 4.1.11
  7. Tera Term (to console into the AP) – you can use any of your choice
  8. TFTP Server

Setting up the AP

Hardware used

Spectrum Analysis 1


Diagram of the setup

This is how all the connections were made, you can chose to use which ever battery and power injector as long as you have the right combination to power up the AP. My battery  had an output of 12V but the Cisco AP takes 48V so i used a cigarette lighter to 110v connection and used a power injector – PowerDsine 3501G

 Spectrum Analysis 2

Get the image from Cisco download software section for the AP model you are using, I used Cisco 3502I

Change the IOS file name to ap3g1-k9w7-tar.153-3.JC.tar to ap3g1-k9w7-tar.default

Set up your Laptop with IP of /24

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Setup TFTP server and Point it to the folder where you have stored the File.

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Turn the AP on by plugging the Ethernet cable from the power injector to the AP Ethernet port, Press and Hold the Mode button behind the AP, release it after 30-40 seconds.

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AP looks for the .default file and starts to download the IOS

Once the AP is in the Autonomous image, add IP address of to the BVI 1 interface

Ap>en  (default password is Cisco)

Config t

Interface BVI 1

IP address

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Now you can change your Laptop IP address to Network (I picked /24)

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Open browser and go to

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Default Username Password admin/Cisco 

Goto Network ->network interfaces and Click on Radio-802.1N 2.4GHz

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Click on Settings and Enable Spectrum

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Do the same for Radio-802.1N 5.0GHz. At this point you have both the Radios set up to be used as Spectrum Analysis

Installing and setting up Cisco Spectrum expert

Download the setup for Spectrum Expert from Cisco website (I used the latest Version 4.1.11)

Install the Sofware with all default settings, once installed launch the software

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Open your console program and console into your AP console and type Spectrum Status

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This will give you the NSI key that is needed for the Cisco Spectrum expert

Once the Cisco Spectrum Expert Launches Put in the IP of the AP and the Key. Also select the Radio you want to use. You can also launch two instances of Spectrum expert and have both 2.4 and 5 GHz running.

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Now you have your Spectrum Expert running with the AP monitoring the Spectrum.

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Open two instance of Cisco Spectrum expert one for each band (2.4 and 5 GHz) and now you can monitor both your bands for any RF activity.

Importance of Spectrum analysis

Any time there is an issue with a Wi-Fi, best is to start trouble shooting at Layer 1. Unfortunately the NIC does not give you an overview of the layer 1, for this you need a Spectrum analyses card or a Cisco Clean Air AP as we used in this case. Since the frequencies used by Wi-Fi 2.4 and 5 GHz both are unlicensed they are also used by other devices such as Baby monitors, cordless phones, blue tooth, etc. Some of these devices have a minor affect and some could impact the Wi-Fi in a huge way.

Example: To generate some non Wi-Fi traffic that will affect the performance I turned on a Baby monitor and you can see how it takes almost all the channels on 2.4 GHz and as soon as it was turned off the interference went away.

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Duty Cycle With baby Monitor

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Duty Cycle Without baby Monitor

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This setup gets heavy to carry around as in my case this was about 3 Kgs. Also since the AP runs in a remote sensor mode it does not display the WLAN’s in the area but for that i was using my Ekahau to give me that view.

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Two great blogs that I read were



I would strongly recommend reading these and also tune into clear to send Pod cast and listen to Rowell and François discussing Spectrum Analysis

Last thoughts

I have tested out the trail new Chanalyzer as they are kind enough to do trial for 7 Days. If my company allows I would love to buy a copy of it as its portable and much more practical to carry around. They also have a additional feature that Chanalyzer lets you connect to a Cisco clear Air AP.

Also netscout has a great product Spectrum XT check that out.

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